8 Million Rats for 8 Million New Yorkers? Data Suggest a City Legend Is Flawed

Jonathan Auerbach, shown in Manhattan on Wednesday, calculated that some two million rats live in the city.Credit
Richard Perry/The New York Times

For some years, statistics have suggested that New York City was getting safer, cleaner and more expensive. Now there are numbers that suggest the city may not be home to as many rats as New Yorkers might have thought — or liked to boast about to visitors.

Jonathan Auerbach, a 26-year-old statistician studying for a doctorate at Columbia University, recently won a competition sponsored by the 180-year-old Royal Statistical Society of London. Mr. Auerbach claimed the prize with a paper in which he made the case that there are far fewer rats in the city than almost anyone had assumed.

About six million fewer.

By Mr. Auerbach’s calculations, the rat population is a mere two million, give or take 150,000.

In arriving at that total, he debunked the long-perpetuated idea that there was one rat for every person in the city. New York’s population stood at 8,405,837 as of July 2013, according to the Planning Department. But in his paper — published in the statistical journal Significance under the title “Does New York City Really Have as Many Rats as People?” — he called the one-person, one-rat idea “an urban myth.”

It is enough to make one wonder what might be next. Perhaps Mr. Auerbach’s work will provide David Letterman with fresh material. Mr. Letterman joked about encountering “rats as big as cats” after he moved to the Ed Sullivan Theater in 1993. Or perhaps Mr. Auerbach’s paper will provide a plot twist for the vampire-centric cable television drama “The Strain.” The show already has a rat exterminator in its cast.

The Department of Health and Mental Hygiene’s approach to counting rodents involves the use of what it calls “rat indexing” to inspect multiple properties in areas on which it concentrates. The agency tells owners to hire exterminators if its inspectors find evidence of rats on their premises, and can issue violation notices if follow-up inspections indicate a lack of compliance. It also holds training courses for building superintendents, property managers, homeowners and even interested tenants.

The health department says its efforts have paid off. “We have seen an overall decrease in the number of active rat signs throughout New York City,” Levi Fishman, the deputy press secretary for the department, said in an email.

How much of a decrease? Mr. Fishman said that “there are no scientific methods for being able to accurately count the number of rats in New York or any large city.” Similarly, Kevin Ortiz, a spokesman for the Metropolitan Transportation Authority, which is well steeped in the battle against rodents, said it had never quantified the rat population living in the subway system.

Photo

A New York rat in one of its natural habitats, the Times Square subway station.Credit
Hiroko Masuike/The New York Times

Mr. Auerbach, who was a fiscal analyst for the City Council and for the New York City Labor Market Information Service at the City University of New York before enrolling at Columbia, acknowledged in his paper that conducting a rat census posed significant challenges for a statistician. “Animals are terrible survey respondents,” he noted dryly.

He faced other methodological problems. He could not capture a large number of rats, mark them for identification purposes, release them, capture a second batch and count the number of marked rats in that batch. “Unfortunately,” he wrote, the health department “is unlikely to approve a large-scale rat-releasing experiment (I know, because I asked).”

Instead, Mr. Auerbach relied on information about rat complaints gathered from 311 call logs, which is publicly available on the city’s data portal. He correlated the location of each reported sighting to the corresponding building lot and concluded that there were 40,500 rat-inhabited lots in New York, or about 4.75 percent of the city’s 842,000 lots.

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“We know that 40 to 50 rats belong to a typical colony,” he wrote. Assuming — “quite generously” — that the larger number was reasonable, he put the rat population at 2.02 million.

The idea that the rat population is smaller than eight million is not news, according to Robert Sullivan, who wrote the book on rats — literally. The title was “Rats: Observations on the History and Habitat of the City’s Most Unwanted Inhabitants.”

“Anybody who knows anything about rats knows there aren’t eight million rats,” Mr. Sullivan said in a telephone interview on Wednesday. “but anybody who knows anything about rats knows that everybody loves the idea of eight million rats. The one-rat-per-person scenario is too good.”

Even so, he noted that the theory was disproved after World War II by the animal behaviorist David E. Davis, who trapped rats in apartments in East Harlem and estimated that, at the time, the city was home to one rat for every 36 people.

Mr. Auerbach’s paper was one of three finalists judged by a panel of professional statisticians. He was invited to speak at the society’s annual conference in Sheffield, England, in September.

The audience, he said, “had a lot of questions about assumptions — do my assumptions hold given the data set?

“At the end, I gave a challenge: ‘Let’s see someone do better.’ I didn’t try to sell the work as a biologist understanding a species. We’re all taxpayers. We pay for services. The government is releasing data. How can we use the data to engage the agencies and talk about the services they provide?”

A version of this article appears in print on November 6, 2014, on Page A23 of the New York edition with the headline: 8 Million Rats for 8 Million New Yorkers? Data Suggest a City Legend Is Flawed. Order Reprints|Today's Paper|Subscribe